The present invention relates generally to microwave waveguide switches, and more particularly to an improved arrangement for a microwave waveguide switch that utilizes cableless waveguide to coax transitions and a standard coaxial relay. This arrangement combines the advantages of relatively low insertion loss and relatively fast switching speed to provide a high performance microwave waveguide switch at significantly lower cost.
Today's communications systems are requiring higher and higher operating frequencies in view of the crowded radio spectrum at the lower, well established frequencies at VHF, UHF, and low microwave frequencies. In implementing a microwave communications system at a higher microwave frequency, for example at 23 Gigahertz (GHz), the designer is often confronted with the need for a waveguide switching element to implement a transmit-receive (T/R) switch, or similar switch permitting redundant switch-over of a backup component into or out of a waveguide or microwave transmission line. While such T/R switches are readily implemented at VHF or UHF frequencies utilizing relays having coaxial connections, such switches, while exhibiting switching speeds of about 10 milliseconds, are nevertheless more difficult to implement at microwave frequencies.
A first known group of microwave waveguide switches is implemented in either manual or electromechanical switching arrangements in which a relatively large inertial mass is rotated within a cylindrical portion of a waveguide. Such waveguide switches, designed for use in the standard waveguide millimeter bands from 26.5 to 220 GHz, exhibit slow switching speeds, on the order of 50 to 100 milliseconds, and have low loss, generally not above 0.2 decibels (dB). However, the cost of such units is generally in the $700 to $900 range. This cost is significantly higher than the cost of coaxial relay switches, which constitute a second known group of microwave switches, most of which are in the $100 price range.
For midrange microwave frequencies on the order of 23 GHz, neither of the above types of conventionally known transmission line switches is suitable for a microwave communications system requiring low loss, relatively fast switching speed, and reasonable cost. Furthermore, attempting to utilize coaxial relays in conjunction with coaxial cables coupling to and from the relay, the designer is faced with very rapidly rising insertion loss due to the inefficiency of the coaxial cable at such microwave frequencies.
A third known alternative utilizes solid state PIN diodes to implement an electronic switch. PIN diodes are so named because of their structure, which allows them to handle radio frequency (RF) energy with low distortion. These diodes have three layers including a P layer, an intrinsic layer, and an N layer. Such solid state switches have loss, however, in excess of 2.0 dB. While the PIN diode switch exhibits a switching time under 1 millisecond, it falls in the $500-$1,000 price range.
One known improvement describes an ultra-high frequency switch having a plurality of terminals and a moveable conductor for establishing and interrupting inter-connection between the terminals and which is actuated by a solenoid having a coil. This moveable conductor utilizes an elongate and flat center conductor in a strip line configuration to make contact with the plurality of terminals and utilizes an impedance matching arrangement for matching between this strip line and the terminals. Although such an arrangement appears to offer significantly shorter switching time and small size construction, several drawbacks are apparent.
The first disadvantage is that the center conductor and drive rod assembly requires precision alignment and mounting in order to insure non-binding movement of the drive rod. Proper alignment of the center conductor with the portions of the waveguide to strip line transitions is also required.
A second disadvantage is that the region immediately surrounding the center conductor, which is required for its movement, also provides a leakage path for microwave signals entering at one waveguide terminal to leak over or loosely couple to the second waveguide terminal when the waveguide switch is in the OFF condition. Although no specific quantity is given for the isolation achieved by this arrangement, most microwave communications systems applications require isolation between ports of a suitable waveguide switch to exceed 40 dB.
A second known arrangement utilizes an electroexpansive element to actuate a first moveable center conductor alternately with a second moveable center conductor in order to connect a stationary common contact to either of a first or a second stationary selection contact. This arrangement describes a single-pole double-throw (SPDT) switch with coaxial connections having moveable center conductor strip line segments similar to the first known arrangement. This second known arrangement is able to achieve the SPDT switch structure with only one actuator, but suffers from the previously mentioned disadvantages related to the internal structure thereof. Each of the disadvantages listed above, therefore, leads to serious compromises in microwave communication system performance, all of which are undesirable and wasteful.
Accordingly, there exists a need for an improved switching element for waveguide which combines the advantages of relatively low insertion loss and relatively fast switching speed and having a simplified assembly with few moving parts in order to enhance the reliability and electrical performance thereof. There exists a further need to provide the above mentioned waveguide switching element which is easily repaired in the event of electrical or mechanical failure therein.